It is known practice for selected gas turbine engine components, especially in the combustor turbine interface, to be internally air cooled by a supply of air bleed from a compressor offtake. Such cooling is necessary to maintain combustor component temperatures within the working range of the materials from which they are constructed.
With reference to EP 2 230 456 A2, FIGS. 1 and 2, a typical gas turbine includes a transition piece by which the hot combustion gases from an upstream combustor as represented by the combustor liner are passed to the first stage of a turbine represented at item 14. Flow from the gas turbine compressor exits an axial diffuser and enters into a compressor discharge case. About 50% of the compressor discharge air passes through apertures formed along and about a transition piece impingement sleeve for flow in an annular region or annulus (or, second flow annulus) between the transition piece and the radially outer transition piece impingement sleeve. The remaining approximately 50% of the compressor discharge flow passes into flow sleeve holes of an upstream combustion liner cooling sleeve and into an annulus between the cooling sleeve and the liner and eventually mixes with the air in annulus. This combined air eventually mixes with the gas turbine fuel in a combustion chamber.
According to EP 2 230 456 A2, FIG. 4, each stub may include one or a plurality of cooling passages disposed substantially surrounding the cooling hole. The cooling passages are preferably oriented at an angle [alpha] relative to an axis (represented by arrow item 54) of the cooling hole in a direction corresponding to a hot gas flow direction (represented by arrow item 56) through the liner. That is, as shown in FIG. 4, the cooling passages are angled relative to axis of the cooling holes, so that the cooling air through cooling passages has at least a directional component in the same direction as the hot gas flow direction through the liner. With the angled cooling passages, it is preferred to include two rows of angled passages through the stub to push the hot gases away from the liner wall. The angle can be any angle up to about 30°, beyond which the air flowing through the cooling passages may have difficulty pushing the hot gases away from the liner wall.
Generally, higher engine gas temperature have led to increased cooling bleed requirements resulting in reduced cycle efficiency and increased emission levels. To date, it has been possible to improve the design of cooling systems to minimize cooling flow at relative low cost. In future engine temperatures will increase to levels at which it is necessary to have complex cooling features to maintain low cooling flows.